All star is not harder than HOF or legend. It does make it much easier to hit so that anyone decent can put enough good swings on the ball to score runs and beat someone that they would not have been able to beat on a higher difficulty. That doesn't make it harder, it makes it more random and there is a key distinction between the two. It's also so easy to hit almost any pitch that it can cause you to become too aggressive but again, that doesn't make it harder.
Believe me, I like playing on HOF or legend much more than all star, but that's because I am good enough to play on those difficulties and most people on this forum are too. However, it would be a bad idea to make everyone under 700 play on HOF because the majority of people that play this game would be striking out 20+ times per game on HOF and you would have a hard time keeping the game going if that was happening.